Electrical conductors with ultrasharp tips have applications as probes for scanned probe microscopy and field emitters for use in scanning electron microscopy (SEM), transmission electron microscopy (TEM) and field emission displays. In probe microscopy, the sharpness of the tip affects the lateral resolution. For field emission, the sharpness of the tip affects the electric field at the tip.
A variety of techniques have been reported for producing sharp tips on electrical conductors, including electrochemical etching, chemical vapor deposition or electron beam deposition onto previously sharpened tips, and ion sputtering. Electrochemical etching is a common technique used to produce sharpened tips on wires of tungsten and other materials. Typically, the radius of curvature of the apex of the tip is about 1 micron or less. Electrochemically sharpened tungsten tips typically have an oxide layer present on the tip surface.
Several ion sputtering techniques have been described in the scientific and patent literature. Biegelson et al. (1987, Appl. Phys. Lett, 50(11) 696) report a technique in which a beam of energetic ions is directed towards an electrochemically etched tungsten tip at an angle with respect to the tip axis. The tip is then rotated within the ion beam, resulting in sputter removal of the oxide layer and reduction of the radius of curvature at the tip apex. U.S. Pat. No. 6,329,214 to Hattori et al, report ion milling of noble metal field emitters with an ion beam incidence angle of 30-60 degrees relative to the substrate normal direction.
Axial incidence ion beam sputtering techniques have also been reported. U.S. Pat. No. 5,993,281 to Musket describe sputtering by high-energy (30 keV and higher) ions incident along or near the longitudinal axis of a field emitter to sharpen the tip with a taper from the point, or top end, down the shank of the emitter. The process is reported to sharpen tips down to radii of less than 12 nm with an included angle of about 20 degrees. U.S. Pat. No. 6,329,214 to Hattori et al. report ion milling of emitters made of conductive material other than a noble metal with an ion beam incidence angle of zero degrees relative to the substrate normal direction. Kubby and Siegel (1986, J. Vac. Sci. Technol. B 4(1), 120) report ion milling of electropolished tungsten and iridium wire targets; the target was electrically isolated from the target chamber and mechanically rotated and the beam energy was in the interval 3-15 keV. Morishita and Okuyama (1991, J. Vac. Sci. Technol. A 9(1), 167) report sharpening of monocrystalline molybdenum tips with Ar+ or Xe+ ions focused into a beam approximately 350 microns across. Hoffrege et al. (2001, J. Appl. Phys., 90(10) 5322) describe variation of the angle between the ion beam and the macroscopic tip.
Focused ion beam (FIB) milling techniques have also been reported. Typical beam diameters (full width at half maximum) are from about 5 nm to about 1 μm. Vasile et al. describe FIB milling of electrochemically etched W and Pt—Ir tips via a three stage process (Vasile et al., 1991, Rev. Sci. Instrum., 62(9), 2167; Vasile et al., 1991, J. Vac. Sci. Technol. B 9(6)). Formation of microtips having radii of curvature between 4 nm and 30 nm was reported (Vasile et al., 1991, Rev. Sci. Instrum., 62(9), 2167). U.S. Pat. No. 5,727,978 to Alvis et al. describes FIB milling of platinum deposited on an electron beam emitting filament.
Self-sputtering sharpening techniques are also described in the scientific literature. As reported by Schiller et al. (1995, Surface Sci. 339 L925-930), electrochemically etched tips are placed in within an ambient neon environment and a high negative voltage is applied to the tip. Under such a voltage, electrons are emitted directly from the tip, impacting and ionizing surrounding neon ions. These positively charged neon ions are attracted to and sputter the tip. The sputtering process results in “necking” and then “decapitation” of the tip.
There remains a need in the art for methods for producing nanometer-scale conducting tips, especially methods which are self-limiting and capable of sharpening more than one tip at a time.